<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
	*{padding:0; margin: 0}
	body{width: 100%; height: 500px; border: 1px solid red }
	.box{width: 200px; height: 200px;border-radius: 50%; background: gray; margin:0 auto 0 auto; animation: move 2s ease-in infinite ;}
	@keyframes move{
		0%{margin-top: 0;}
		50%{margin-top: 280px;}
		100%{margin-top: 0px;}

	}

</style>
</head>
<body>
		<div class="box"></div>
</body>
</html>